Re: Carbon on the outside of the case neck after firing?
I strive for low ES in my reloading plan.
With this there are a few adjustments from common:
-Fastest appropriate powder -that fills a case
-Best performing primer and best primer striking
-Long enough barrel for powder burn
-Matched reloaded case H20 capacities
-Smallest neck clearances practical
-Tightest chamber end to trim clearance practical
-Consistent neck tension
With testing in the past I could call a hit to ES with a look at neck sooting, before checking the chrono. And I eventually managed it with more consideration of clearances left with trim length.
Less chamber end clearance meant less sooting, meant less ES with my cartridges & loads.
This was an easy adjustment for me because 'matched reloaded case H20 capacities' took me away from FL sizing, which took me away from chasing trim lengths anyway.
I started setting the fireformed chamber end clearance as close as I could, while not under 5thou, and a little bit of trimmed neck length variance from this(~5thou) did not matter to ES.
Being this close I still look as a habit for any shine to mouths on ejection. This is a sign I'm getting carbon buildup from chamber end(time to clean), or the brass has stretched to chamber end(which is dangerous).
Now & then I see the carbon sign, but I haven't had to actually re-trim necks yet.
It's just one factor, but no matter the cartridge or load, I wouldn't act on generalizations about it.
It's like most anything, you should do as well as practical with it.